Composite wood products have found great favor in various industries. Whether bonded or laminated, composite wood products often exhibit superior properties to wood of similar dimensions. Composite products often are stronger, exhibit better resistance to degradation and failure, and are more cost-effective than wood alone.
Diverse composite wood products have gained acceptance. Particleboard, fiberboard, waferboard, OSB, plywood, laminated veneer lumber (LVL) and laminated beams, made from various wood sources including wood particles, fibers, and/or veneers, are but a few of the composite wood products that have become widely accepted in industry. Such products are formed from wood pieces of appropriate size and form. These wood pieces are bonded together with an adhesive to form the composite wood product.
A number of factors govern the suitability of any particular adhesive for its use in wood composites. For example, because wood is porous and can have varying moisture content, an adhesive should be able to penetrate the wood to a degree sufficient to form the desired bond while accommodating the moisture content of the wood. Also, various varieties of wood have differing grain structures that affect the bonding capability of the wood with adhesive. The adhesive must not only penetrate the wood, but also dry or cure in an appropriate period. Drying too quickly will limit wood penetration and ability to re-position the wood pieces, if necessary. Drying too slowly will cause production delays and thus increase cost.
The ability to form a shape-retaining “green” composite product in which the adhesive has not yet cured, also is important in many applications. In particular, this property requires an adhesive that has sufficient “tack” (i.e., the ability to retain the shape of the product without completely curing). Tack enables adhered particles or layered structures to stick together, rather than move individually, thereby allowing a manufacturer to remove green board from a shape-retaining container before the adhesive is fully cured. This improves composite production rate and eases manufacturing procedures, relative to cases where the adhesive exhibits little or no tack. An adhesive that has tack before complete curing also advantageously allows for re-arrangement of the wood pieces (e.g., wood layers) relative to each other while essentially retaining the form originally imparted. Sufficient tack therefore permits the wood pieces to be packed more efficiently, for example, or to be re-positioned before complete curing of the adhesive.
Finally, a suitable adhesive must accommodate not only the type of wood and wood pieces to be bonded, but also the ultimate use of the composite. For example, composites that are normally exposed to water (e.g., those in outdoor service) have different adhesive requirements than those used in dry locations. Adhesives also should fill any gaps between wood pieces during curing and have a viscosity that allows for rapid application with typical processing equipment.
To accommodate the above considerations and provide desirable properties and characteristics, a number of adhesive systems for wood composites have been developed. For example, aldehyde-based resins formed generally from the reaction product of an aldehyde and an aldehyde-reactive compound (phenol-formaldehyde, urea-formaldehyde, acetone-formaldehyde, melamine-formaldehyde, etc.) have long been recognized to possess favorable tack-building capability, curing rates, and bonding strength. See, for example, U.S. Pat. No. 4,915,766. Recently, there has been increasing interest in reducing emissions during the manufacture of building materials and also from finished building materials, and this has spurred interest in non-formaldehyde based wood adhesives.
Isocyanate-based resins (which, although made from aldehydes, do not emit measurable amounts of aldehydes) are employed in the production of OSB and other composite wood products, as described, for example, in U.S. Pat. No. 5,407,980. Isocyanate resins are known to be particularly resistant to water, and therefore especially useful in the manufacture of exterior products or other products intended for exposure to water. Isocyanate-based resins) however, are not completely satisfactory for the high-speed production of composite wood products, because such adhesives, in their uncured state, have essentially no tack. In applications such as caulless particleboard manufacturing, isocyanate-based resins are wholly unusable due to the absence of sufficient tack required to hold wood particles together as they are conveyed from the forming station to the press. Furthermore, the manufacture of composites using isocyanate-based resins normally requires in-plant emissions controls. Isocyanate-functional resins are also relatively expensive.
In view of the above-noted concerns relating to petroleum-based adhesives, the art has recognized the potential attractiveness of alternatively using protein-based adhesives, in order to reduce the usage of petrochemicals and potentially reduce manufacturing process emissions as well as product emissions. For example, protein glues, such as various casein, blood, and other compositions, have been investigated and are discussed in detail, for example, in the background section of U.S. Pat. No. 6,306,997. Another reason why protein-based adhesives are desirable is that they are made from a renewable resource. Adhesives may be made from protein products (e.g., wheat- and corn-based products), including vegetable powder or flour. Animal proteins have also been used as adhesives.
Adhesives made from soy protein products, particularly hydrolyzed soy protein, are also known. For example, U.S. Pat. Nos. 6,365,650 and 6,231,985 describe an adhesive suitable for preparing wood composites formed by combining an isocyanate prepolymer with hydrolyzed soy protein. Soy protein hydrolyzate, however is unsuitable as an adhesive in certain applications for a number of reasons. It is viscous and dries quickly, often before suitable bonds can be made with the wood. Thus, composite wood products made with soy protein hydrolyzate adhesive often have low strength. Moreover, the use of soy protein in general can render composite products unsuitable for uses involving contact with water or moisture.
The art has attempted to address these and other inherent disadvantages of protein-based adhesive formulations by incorporating various additives. For example, U.S. Pat. No. 5,344,871 describes binders for wood fillers or boards. The binders are principally protein but may be improved with an additive that enhances cross-linking, such as one which forms amide cured epoxys and polyamide.
Similarly, U.S. Publication No. 2004/0089418 describes an adhesive composition for lignocellulosic composites. The adhesive comprises the reaction product of a soy protein or lignin (phenolic polymer) and at least one substantially formaldehyde-free curing agent. The curing agent includes at least one amine, amide, imine, imide, or nitrogen-containing heterocyclic functional group that can react with at least one functional group of the soy protein or lignin.
U.S. Publication No. 2004/0037906 A1 describes reacting a soy protein-based adhesive with a compound that introduces additional phenolic hydroxyl, amine, and/or thiol functional groups into the soy protein structure to provide improved strength and water resistance.
The soy protein additives specifically disclosed in the above-mentioned U.S. Publication No. 2004/0089418 include AMRES® (Georgia-Pacific Corporation) and KYMENE® (Hercules, Inc.), which are well known in the art as useful for imparting wet strength to paper and paper products. The use of such wet strength resins in minor amounts in other applications is described, for example, in U.S. Pat. No. 4,210,692. This patent describes a furnish comprising wood fiber, polymer, mineral fiber, size, and a small amount (0.1-2% by weight) of wet strength agent. The furnish is made into a sheet that is adhered to and forms the outer layer of a wood article.
Even in view of the above disclosures, there is an ongoing need in the art for new adhesives for composite wood products. Especially desired are adhesives that have good tack and thus provide for shortened processing time and contribute to reduced production costs. The adhesives should also exhibit the good bond strength, quick curing properties, and other desirable characteristics traditionally associated with petroleum-based resins. Moreover, such adhesives should emit relatively little or no volatile compounds upon curing or undergoing other processing steps. Finally, the adhesives should also be cost-effective and water-resistant.